This invention relates to an objective lens unit for an optical pickup used for a disc driving device illuminating the laser light on an optical recording medium for recording and/or reproducing the information, an optical pickup employing this objective lens unit and to a disc driving device employing this objective lens unit. Specifically, this invention relates to an objective lens unit capable of converging the laser light close to the diffraction limit on a recording surface of the optical recording medium, an optical pickup employing this objective lens unit, and to a disc driving device employing this optical pickup.
As a recording medium for information signals, a non-contact optical recording medium, having a high recording density, and adapted for illuminating the laser light on the recording surface for recording and/or reproducing the information, is finding widespread application. Of the various types of the optical recording medium, a disc-shaped optical recording medium, referred to below as an optical disc, is used most extensively because of ease in retrieving the information recorded thereon.
An optical disc has concentric recording tracks or a spirally-shaped recording track. The separation between neighboring recording tracks or neighboring turns of the recording track, or a track pitch, is approximately 1.6 xcexcm in the case of, for example, a Compact Disc (CD). In a DVD (Digital Video Disc/Digital Versatile Disc), developed more recently, this track pitch is of a narrower value of 0.74 xcexcm, which means appreciably improved recording density of the information signals. For radiating the laser light on an optical disc in which the track pitch is reduced to improve the information recording density for recording and/or reproducing information signals, such as DVD, it is necessary to form a beam spot, smaller in size than for an optical disc with a larger track pitch, on its recording surface.
The diameter of the beam spot converged by the objective lens unit is proportionate to the to the design diameter of the laser light and inversely proportionate to the numerical aperture NA of the laser light. Thus, for diminishing the diameter of the beam spot illuminated on the optical disc, it is necessary to increase the numerical aperture of the objective lens unit and to reduce the wavelength of the laser light.
On the other hand, since a high energy laser light is required for recording the information by a phase change system, or the like. For reducing the laser noise by the reflected laser light, the driving power is varied by a method of superposing the high frequency on the voltage or the driving current of the laser light emitting device to vary the laser light wavelength in a short period. Thus, in an optical pickup adapted for illuminating the coherent laser light on the optical disc, chromatic aberration ascribable to wavelength variation on the order of several nm is produced to enlarge the beam spot on the optical disc.
Meanwhile, the optical pickup is provided with an objective lens unit 201 for converging the laser light 200 on the signal recording surface of the optical disc. This routine objective lens unit 201 is formed by a single lens molded of glass and has an aspherical surface 201a having high light converging power, as shown in FIG. 1.
Referring to the charts of FIGS. 2A, 2B and 2C, showing the spherical aberration astigmatic aberration and distortion aberration, respectively, it may be seen that the chromatic aberration on the order of xc2x10.6 xcexcm/nm is produced against wavelength variation of xc2x12 nm, even with the use of the aspherical surface 201a. In the aberration charts of FIGS. 2A to 2C, solid lines, dotted lines and chain-dotted lines depict the values for 405 nm, 403 nm and 407 nm, respectively, whereas, in FIG. 2B showing the astigmatic aberration, thick and thin lines depict a sagittal image surface and a tangential image surface, respectively.
For recording the information on an optical disc, in which the track pitch of the recording track is narrowed for increasing the recording density, it is desirable to converge the laser light close to the diffraction limit by the objective lens unit to form a beam spot of a smaller diameter. With the single lens type objective lens unit 201, produced as conventionally by casting glass with the aspherical surface 201a, it is difficult to converge the laser light close to the diffraction limit because of the chromatic aberration distortion aberration.
In view of the above depicted status of the art, it is an object of the present invention to provide an objective lens unit that may be used with advantage for an optical pickup used for recording the information on an optical recording medium, in which the track pitch is narrowed to raise the recording density, and for reproducing the information recorded on the optical recording medium. It is a more specific object of the present invention to provide a small-sized objective lens unit that is capable of effectively correcting the chromatic aberration and converging the light beam close to the diffraction limit.
It is another object of the present invention to provide an optical pickup employing an objective lens unit that is capable of effectively correcting the chromatic aberration and converging the light beam close to the diffraction limit, and an optical disc employing this optical pickup.
For accomplishing the above object, the present invention provides an objective lens unit for an optical pickup including a resin layer having, sequentially from an object side, a first surface as an aspherical surface and a second surface as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface, and a lens of glass having the second surface as an aspherical surface and a third surface as an aspherical surface. The lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of the reference wavelength, with the numerical aperture of the lens unit being not less than 0.8. With this objective lens unit, the light can be converged on the image surface close to the limit of diffraction.
According to the present invention, the base surface defining the objective lens unit necessarily has a value of the aspherical coefficient, so that the diffraction type lens can be formed to an optically well-balanced stabilized state.
With the objective lens unit of the present invention, the aspherical surface of the first surface and the aspherical surface of the second surface are of the same base curvature and the same aspherical coefficient, so that the resin layer is of the constant film thickness to increase stability against changes in temperature. With the objective lens unit of the present invention, the resin layer, the first surface or the second surface of which is the diffractive surface, is of the transmission phase type, with the first surface or the second surface being of the blazed shape, so that the numerical aperture of the lens unit may be increased without diminishing the radius of curvature of the base surface of the lens unit. Since the first or second surface, as the diffractive surface, is of the blazed shape, it is possible to eliminate diffraction efficiency deteriorating factors, such as non-machinable portions, or transmittance deteriorating factors, such as a serrated shape, during the forming of the diffraction surface.
With the objective lens unit of the present invention, an aperture is formed in the outer rim of the first surface so as to allow usage of only the lens surface portion exhibiting desirable optical properties and so as not to allow usage of the lens rim to suppress production errors to stabilize the performance in assembling the lens unit.
With the objective lens unit of the present invention, in which the design degree of orders is not less than two, it is possible to correct for chromatic aberration more effectively to wine-press the spot diameter of the laser light close to the diffraction limit, while it is also possible to cope with an optical recording medium in which the track pitch is reduced to raise the information recording density.
The single lens of glass, forming the objective lens unit of the present invention, is formed of a vitreous material, having a refractive index of 1.65 or more with respect to the light of a wavelength of 420 nm or less, and hence the single lens of glass is of the meniscus shape to allow miniaturization of the third order aberration.
With the objective lens unit of the present invention, in which the fine micro-irregular shape having a period approximately one-half the reference wavelength and an amplitude approximately one-half the reference wavelength is formed on the first surface or the second surface carrying the diffractive surface, with the micro-irregular shape being finer than the micro-irregular shape of the diffractive surface, the transmittance of the compound surface, that cannot be polished, can be 90% or higher.
With the objective lens unit of the present invention, in which a protective cover having a thickness not less than 0.3 mm is arranged between the objective lens unit and the image surface, and the spherical aberration ascribable to the protective cover is corrected, it is possible to eliminate the effect of the protective cover on the spherical aberration.
For accomplishing the above object, the present invention also provides an optical pickup comprising a laser light emitting device for radiating the laser light, an objective lens unit for converging the laser light on a recording layer of an optical recording medium, a light receiving device for receiving the laser light and an optical component for causing the laser light radiated from the laser light emitting device to incident on the objective lens unit and for causing the laser light reflected by the recording layer of the optical recording medium and transmitted through the objective lens unit to incident on the light receiving device, wherein the objective lens unit includes a resin layer having, sequentially from an object side, a first surface as an aspherical surface and a second surface as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface, and a lens of glass having the second surface as an aspherical surface and a third surface as an aspherical surface. The lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of the reference wavelength, with the numerical aperture of the lens unit being not less than 0.8.
With the present optical pickup, in which the chromatic aberration can be effectively corrected for the wavelength range within several nm about 420 nm or less as a reference, the spot diameter of the laser light can be wine-pressed close to the diffraction limit to cope with the standard for the optical recording medium in which the track pitch is narrowed to raise the information recording density.
With the optical pickup of the present invention, the working distance from the third surface of the objective lens to the image point is set to 0.5 mm or more to avoid the problem of the objective lens unit colliding against the optical recording medium to render it possible to converge the light close to the diffraction limit on the recording layer of the optical recording medium.
With the optical pickup of the present invention, in which a protective cover having a thickness not less than 0.3 mm is arranged between the third surface of the objective lens unit and the image surface, and the function of correcting the spherical aberration ascribable to the protective cover is provided, it is possible to eliminate the effect of the protective cover on the spherical aberration.
For accomplishing the above object, the present invention also provides a disc driving device for recording and/or reproducing the information for a rotating disc-shaped optical recording medium by an optical pickup movable along the radius of the optical recording medium, wherein the optical pickup includes a laser light emitting device for radiating the laser light, an objective lens unit for converging the laser light on a recording layer of an optical recording medium, a light receiving device for receiving the laser light and an optical component for causing the laser light radiated from the laser light emitting device incident onto the objective lens unit and for causing the laser light reflected by the recording layer of the optical recording medium and transmitted through the objective lens unit incident onto the light receiving device. The objective lens unit includes a resin layer having, sequentially from an object side, a first surface as an aspherical surface and a second surface as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface, and a lens of glass having the second surface as an aspherical surface and a third surface as an aspherical surface. The lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of the reference wavelength, with the numerical aperture of the lens unit being not less than 0.8.
With the disc driving device of the present invention, in which the chromatic aberration can be effectively corrected for the wavelength range within several nm about 420 nm or less as a reference, the spot diameter of the laser light can be wine-pressed close to the diffraction limit to cope with the standard for the optical recording medium in which the track pitch is narrowed to raise the information recording density.
By setting the working distance from the third surface of the objective lens to the image point to 0.5 mm or more, it is possible to avoid the problem of the objective lens unit colliding against the optical recording medium to render it possible converge the light close to the diffraction limit on the recording layer of the optical recording medium.
Other objects, features and advantages of the present invention will become more apparent from reading the embodiments of the present invention as shown in the drawings.